To purify water of a basin, a lake or rivers a basin, a lake or rivers of which a problem of water pollution is raised.

Raw water is supplied to a purifying tank and fine bubbles are raised toward the surface of raw water from the bottom part of this purifying tank and waste refuse, org. matter or bacteria in raw water are stuck to these air bubbles to be accompanied by air bubbles and a part of raw water to which waste refuse is stuck to be allowed to overflow from the upper end surface of the purifying tank or filtered by a filter agent such as zeolite to be primarily filtered. This primarily filtered treated water is supplied to a water purifying tank filled with a solid oxygen generator such as calcium peroxide and passed through a purifying tank to be purified. This treated water is supplied to a water purifying tank packed with activated carbon to be further purified. By this constitution, treated water free from waste refuse, having oxygen sufficiently dissolved therein and decolored and deodorized is obtaned.